I. Field
The following description relates generally to communications systems, and more particularly to performing channel estimations while effectively mitigating co-channel interference.
II. Background
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so forth. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP Long Term Evolution (LTE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can concurrently support communication for multiple wireless terminals that communicate with one or more base stations via transmissions on forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-signal-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels where NS≦min{NT, NR}. Generally, each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system also supports time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows estimation of the forward link channel from the reverse link channel. This enables an access point to extract transmit beam-forming gain on the forward link when multiple antennas are available at the access point.
Wireless systems transmit pilot signals from base stations to receivers in order to facilitate communications. One important aspect that pilot signals are employed for is channel estimation at the respective receivers. In general, pilot signals can be generated from multiple base stations in a given area or from multiple sectors from a given base station. Such signal transmissions from these multiple transmission sources often interfere with each other. Thus, mechanisms have been devised to mitigate such interference. In one case, signals from different base stations or sectors can be encoded on different frequencies to mitigate interference between sources. Unfortunately, the number of frequencies for transmitting pilot signals is limited. Thus, other solutions have evolved. In another case, signals from different base stations or sectors are transmitted as an orthogonal sequence of three vectors, where each vector represents a different base station or sector. Utilizing this scheme, processing for a single sector requires processing all three vectors concurrently in order to cancel out the effects of cross channel interference. Concurrent vector processing unfortunately does not provide the desired detailed channel estimate information for any particular sector or base station since the computation is an amalgam derived from three sectors. Thus, it would be desirable to be able to mitigate interference between differing sectors and stations while being able to analyze a particular sector or station from other sources in order to effectively perform channel estimation.